In the treatment of cellulose, such as in the digestion of wood chips in paper manufacture, the wood chips are treated under high pressure with steam and a chemical solution known as cooking liquor in a unit called a digester. Conventionally, after treatment in the digester the cellulose, cooking liquor and steam are discharged, usually on a continuous basis, into a unit generally referred to as a blow tank.
U.S. Pat. No. 2,870,009 discloses apparatus for recycling steam from a low pressure blow tank back to a steaming vessel for pretreatment of the wood chips being fed to a digester. It is stated in the patent that the pressure in the blow tank "is preferably not very high and may be of the order of 1 atmosphere guage". A special, two valve discharge device is utilized to prevent the tank from being exposed to high pressures from the digester, and as a result of the pressure drop of the pulp coming from the digester, gases, mostly steam, are given off from the pulp as it enters the blow tank.
While the apparatus disclosed in the above noted U.S. patent does utilize to some degree the low pressure steam from the blow tank, it will be apparent that no attempt is made to control and maintain high steam pressures in the blow tank, and in fact the opposite approach is taken, that is, steps are taken to prevent high steam pressure in the blow tank. As a result valuable high pressure steam which might be used in various heat recovery units is lost as well as an opportunity to provide significant additional fiber treatment in the blow tank.